1. Field of the Invention
The present invention relates to a semiconductor laser device with a phase shift region having a polarization dependency suitably usable as a light source for optical communications or the like, such as a distributed feedback (DFB) semiconductor laser capable of switching a polarization mode of its output light between two polarization modes (typically, transverse electric (TE) mode and transverse magnetic (TM) mode) depending on its driven condition. The present invention also relates to an apparatus or system using the laser device.
2. Related Background Art
In recent years, optical communication and optical information processing have been earnestly studied to cope with a rapid increase in information handling capacity due to development of multimedia and the like. A dynamic-single-mode device with a narrow spectrum has been needed to serve as a light source for those optical communications and for information processing. For such purposes, DFB semiconductor lasers and distributed Bragg reflector (DBR) semiconductor lasers have been developed and studied. Japanese Patent Application Laid-Open No. 2(1990)-159781 (JP ""781), for example, discloses a polarization switchable laser which can emit an intensity-modulated signal with a high extinction ratio and can serve the purposes described above. In this device, a pumped condition in its portion is changed to perform the switching of its oscillation polarization mode.
FIG. 1 illustrates the polarization switchable device. FIG. 1 is a cross-sectional view taken along a laser resonance (cavity-axial) direction of the device. The structure includes a light guide layer 1102, an active layer 1103, a clad layer 1104, and a contact layer 1105 which are laid down over a substrate 1101 of InP. A uniform diffraction grating 1108 is formed at the interface between the light guide layer 1102 and the substrate 1101. The contact layer 1105 is divided into three portions along the cavity-axial direction. Bias electrodes 1106a and 1106b and a control electrode 1107 are respectively deposited on the three portions of the contact layer 1105. The control electrode 1107 is formed on a region for shifting the phase of an equivalent refractive index. A common electrode 1109 is formed on the bottom surface of the substrate 1101. The control electrode 1107 and the bias electrodes 1106a and 1106b are electrically separated from each other, so a current can be independently injected through the control electrode 1107. In the device of FIG. 1, the current injected into the phase shift region through the control electrode 1107 is changed or modulated under a condition under which appropriate bias currents are injected through the bias electrodes 1106a and 1106b. Thus, the equivalent refractive index is partly changed, and the shift amount of the equivalent refractive index is controlled for each of the two different polarization modes. Consequently, the relation between threshold gains for the two modes is changed and polarization switching is performed.
In the above proposal, the polarization modulation system itself is an advantageous one. However, in an ordinary DFB laser, oscillation in the TE mode is dominant over that in the TM mode, so polarization mode contention is difficult. JP ""781 discloses no specific solution for this problem.
Further, in a DFB laser lacking built-in phase shift section, oscillation occurs at wavelengths at either or both ends of its stop band rather than at its Bragg wavelength in a single mode, due to adverse influences of fine unevenness in the diffraction grating and the phase at the end facet. In the above proposal, although the pumping condition is partially varied to introduce the phase shift, such phase shift due to the control of current injection is unstable and it is hence hard to achieve stable single-mode oscillation. Furthermore, the current for attaining the single-mode oscillation and the current for achieving the polarization switching vary among individual devices due to influences of the end-facet phase and so forth.
It is an object of the present invention to provide a semiconductor laser, such as a polarization switchable distributed feedback semiconductor laser, which includes a phase shift region with a polarization dependency such that light in a polarization mode influenced by a xcex/4 phase shift can be stably oscillated in a single longitudinal mode, an optical transmitter with the laser, and an optical transmission system or method using the laser.
It is a second object of the present invention to provide a semiconductor laser, such as a polarization switchable distributed feedback semiconductor laser, which can suppress unfavorable phenomenon, such as hole burning due to extreme concentration of light on a part and can be fabricated by a simple process without needing a complicated process required for the fabrication of a conventional xcex/4 phase shift diffraction grating.
It is a third object of the present invention to provide a semiconductor laser, such as a polarization switchable distributed feedback semiconductor laser, which can effect stable polarization-mode contention and effect a single-mode oscillation in each of the TE mode and the TM mode.
The objects of the present invention are achieved by the following lasers, transmitters and optical communication systems or methods.
A distributed feedback semiconductor laser of the present invention includes a waveguide with an active layer and a diffraction grating, which extends along a cavity-axial direction and is defined such that propagation of light in two different polarization modes is permitted in the waveguide; and a first phase shift region formed in the waveguide. The first phase shift region extends along the cavity-axial direction and has a polarization dependency that an effective refractive index for propagation light of the first phase shift region differs from an effective refractive index for propagation light of a region of the waveguide other than the first phase shift region such that a phase shift of a quarter wavelength of the propagation light is created for one of the two polarization modes and a phase shift of a half wavelength of the propagation light is created for the other of the two polarization modes in the first phase shift region.
The laser of the present invention can be typically constructed to act as a DFB semiconductor laser which can switch or modulate its oscillation polarization mode (in this specification, xe2x80x9cswitchxe2x80x9d, xe2x80x9cswitchablexe2x80x9d and the like are used in a broad sense including a modulation wherein the polarization mode is changed at a relatively high speed), but its structure is not limited thereto. For example, the laser of the present invention can also be constructed as a single-mode tunable semiconductor laser which can change its wavelength while its polarization mode remains unchanged, or a single-mode semiconductor laser which can stably oscillate in one polarization mode in a single mode.
Specifically, the following configurations of three types can be adopted based on the above fundamental structure.
In a first configuration, the laser can oscillate light in two different polarization modes of TE mode and TM mode, the active layer generates a larger gain for the TM mode than for the TE mode, and the first phase shift region creates a phase shift of a quarter wavelength for the propagation light in the TE mode and creates a phase shift of a half wavelength for the propagation light in the TM mode.
In this case, nTE, nTM and L are preferably determined such that xcex1=(4xc3x97Lxc3x97nTE+2xc3x97xcexTE)/(4xc3x97Lxc3x97nTM+xcexTM) is satisfied where nTE and nTM are effective refractive indices of the region other than the first phase shift region for the TE mode and the TM mode, respectively, xcexTE and xcexTM are wavelengths of the propagation light in the TE mode and the TM mode, respectively, n1TE and n1TM are effective refractive indices of the first phase shift region for the TE mode and the TM mode, respectively, xcex1=n1TE/n1TM is a ratio between effective refractive indices of the first phase shift region for the TE mode and the TM mode, and L is a length in the cavity-axial direction of the first phase shift region.
In a second configuration, the laser can oscillate light in two different polarization modes of the TE mode and the TM mode, the active layer generates a larger gain for the TE mode than for the TM mode, and the first phase shift region creates a phase shift of a quarter wavelength for the propagation light in the TM mode and creates a phase shift of a half wavelength for the propagation light for the TE mode.
In this case, nTE, nTM and L are preferably determined such that xcex2=(4xc3x97Lxc3x97nTE+xcexTE)/(4xc3x97Lxc3x97nTM+2xc3x97xcexTM) is satisfied where nTE and nTM are effective refractive indices of the region other than the first phase shift region for the TE mode and the TM mode, respectively, xcexTE and xcexTM are wavelengths of the propagation light in the TE mode and the TM mode, respectively, n1TE and n1TM are effective refractive indices of the first phase shift region for the TE mode and the TM mode, respectively, xcex1=n1TE/n1TM is a ratio between effective refractive indices of the first phase shift region for the TE mode and the TM mode, and L is a length in the cavity-axial direction of the first phase shift region.
In a third configuration, the laser can oscillate light in two different polarization modes, a second phase shift region is further formed in the waveguide, the second phase shift region extends along the cavity-axial direction and has a polarization dependency that an effective refractive index for propagation light of the second phase shift region differs from an effective refractive index for propagation light of a region of the waveguide other than the second phase shift region such that a phase shift of a quarter wavelength of the propagation light is created for the other of the two polarization modes and a phase shift of a half wavelength of the propagation light is created for one of the two polarization modes.
In this case, nTE, nTM, L1 and L2 are preferably determined such that xcex11=(4xc3x97L1xc3x97nTE+xcexTE)/(4xc3x97L1xc3x97nTM+2xc3x97xcexTM) and xcex12=(4xc3x97L2xc3x97nTE+2xc3x97xcexTE)/(4xc3x97L2xc3x97nTM+xcexTM) are satisfied where nTE and nTM are effective refractive indices of the region other than the first and second phase shift regions for the TE mode and the TM mode, respectively, xcexTE and xcexTM are wavelengths of the propagation light in the TE mode and the TM mode, respectively, n1TE and n1TM are effective refractive indices of the first phase shift region for the TE mode and the TM mode, respectively, n2TE and n2TM are effective refractive indices of the second phase shift region for the TE mode and the TM mode, respectively, xcex11=n1TE/n1TM is a ratio between effective refractive indices of the first phase shift region for the TE mode and the TM mode, xcex12=n2TE/n2TM is a ratio between effective refractive indices of the second phase shift region for the TE mode and the TM mode, and L1 and L2 are lengths in the cavity-axial direction of the first and second phase shift regions, respectively.
Further, in this case, the active layer is preferably formed to generate an approximately equal gain for each of the TE mode and the TM mode, thereby obtaining a single-mode semiconductor laser which can switch its oscillation polarization mode.
More specifically, the following structures may be adopted.
A shape of the first phase shift region can be different from a shape of the other region of the waveguide to achieve a phase shift action with the polarization dependency. In this case, a width of the first phase shift region may be different from a width of the other region of the waveguide, or a thickness of the first phase shift region may be different from a thickness of the other region of the waveguide, to achieve the phase shift action with the polarization dependency.
The active layer may be a tensile-strained active layer, or a quantum well active layer, to obtain a desired relation between gains for the two different polarization modes.
The active layer may be formed on a side of a substrate with respect to the diffraction grating, or on a side opposite to the substrate with respect to the diffraction grating.
The diffraction grating with different coupling coefficients for the two different polarization modes has a uniform pitch without a phase shift section over an entire length of the diffraction grating, thereby achieving a semiconductor laser with a simple structure which can be fabricated without requiring a complicated process.
Further, a first current injection unit for injecting a current into a region including the first phase shift region and a second current injection unit for injecting a current into a region lacking the first phase shift region may be formed.
A first current injection unit for injecting a current into a region including the first phase shift region and a second current injection unit for injecting a current into a region including the second phase shift region may be formed.
According to another aspect of the present invention, there is provided a light source apparatus including the above distributed feedback semiconductor laser which is constructed as a polarization switchable DFB laser, and a mode selector for selecting a component of a desired mode from a light output from the laser.
According to still another aspect of the present invention, there is provided an optical transmitter including a distributed feedback semiconductor laser which is constructed as a polarization switchable DFB laser, a controller for controlling a light output from the laser in accordance with a transmission signal, and a mode selector for selecting a component of a desired mode from the light output from the laser.
According to still another aspect of the present invention, there is provided an optical communication system for communicating over a light transmission line that transmits a signal from a transmitter side to a receiver side, which includes the above optical transmitter for transmitting light of a signal through the light transmission line, and a receiver for receiving and detecting an intensity-modulated signal transmitted from the laser through the light transmission line.
In those apparatuses and systems, the mode selector is set such that a light component in one of the two polarization modes can be selected.
In the above first and second configurations above, since only one polarization mode is actually used for signal transmission in the polarization modulating system, the structure is formed such that the single-mode oscillation can be achieved at least in the polarization mode to be used for the signal transmission. In addition, the polarization contention enough for the polarization switching can be established. Where the xcex/4 phase shift is imparted only to the TE mode, since most conventional semiconductor lasers oscillate in the TE mode, there can be provided a structure which can oscillate in the TE mode in a single mode, can effect the polarization switching, and is applicable to apparatuses and systems designed for the conventional semiconductor lasers. Where the xcex/4 shift is imparted only to the TM mode, the polarization contention can be readily effected by this phase shift means, even though a gain for the TE mode is larger than that for the TM mode in an ordinary bulk active layer or the like. A desired gain can be further readily obtained by a tensile-strained active layer.
In the third configuration above, either polarization c an be used for the signal transmission, and in this case an active layer, which has approximately equal gains for TE mode and TM mode, is preferably used.
In the above fundamental structure, the single-mode oscillation in a desired polarization mode can be stably achieved with a relatively simple structure. When gains for the two polarization modes are competitive, or an appropriately-designed active layer is used, a polarization switchable laser can be accurately attained. Further, since the length of the phase shift region is quite large (for example, from several tens of xcexcm to 100 xcexcm), hole burning can be suppressed and polarization switching operations can be stabilized.
The operation principle of each of the three configurations based on the fundamental structure will be described.
FIGS. 2A and 2B show the first configuration of the laser of the present invention. As illustrated in FIGS. 2A and 2B, the laser includes a substrate 1, an active layer 2, a light guide layer 3, a clad layer 4, contact layer (portions 5a and 5b), two upper electrodes 6a and 6b, a diffraction grating 7 with a uniform pitch xcex9, a common lower electrode 8, a structurally built-in TE-mode phase shift region 9, and a stripe waveguide 10. A relatively large tensile strain is introduced into the active layer 2 such that a gain for the TM mode is larger than that for the TE mode. The length in the cavity-axial direction of the phase shift region 9 is much larger than a conventional xcex/4 shift section which attains the phase shift employing a xcex/4 deviation of a pitch of the grating. Such a conventional xcex/4 shift section inherently cannot be polarization-dependent.
Where the following notations have respectively the above definitions, the following two relations (especially, relation (1) is indispensable) must be satisfied in the phase shift region 9 in order for the TE mode to receive a xcex/4 shift (xcex/4(2n+1), n being an integer):
(n1TExe2x88x92nTE)xc3x97L=xcexTE/4xe2x80x83xe2x80x83(1),
and
(n1TMxe2x88x92nTM)xc3x97L=xcexTM/2xe2x80x83xe2x80x83(2).
From the two relations, nTE, nTM and L need to be determined such that the following relation (3) is satisfied:
xcex1=(4xc3x97Lxc3x97nTE+2xc3x97xcexTE)/(4xc3x97Lxc3x97nTM+xcexTM)xe2x80x83xe2x80x83(3).
Under the above conditions, a phase shift received by light in the TM mode after a single reciprocative path thereof in the cavity is xcex and the phase of the light after the single reciprocative path coincides with the phase of original light, so the phase shift region 9 does not at all influence the light in the TM mode. Thus, only light in the TE mode effectively receives the phase shift, and hence a stable single-mode oscillation can be achieved in the TE mode. Here, since the TM mode never receives the phase shift action, the polarization switching can be stably achieved when the gain for the TM mode is made larger than that for the TE mode in the active layer to obtain the polarization contention. (When only a stable single-mode oscillation in the TE mode is desired, only a gain for the TE mode has to be sufficient.) Thus, there are structurally provided the phase shift with the polarization dependency for acting on the TE mode only and the gain adjustment for facilitating the polarization contention, so a stable single-mode oscillation in the TE mode and the polarization contention can be obtained. Further, the phase. shift region 9 can be elongated, so adverse influences, such as hole burning from concentrating light in the phase shift portion can be effectively reduced and the operation can be stabilized.
FIGS. 7A and 7B show the second configuration of the laser of the present invention. As illustrated in FIGS. 7A and 7B, the laser includes a substrate 21, an active layer 22, a light guide layer 23, a clad layer 24, contact layer (portions 25a and 25b), two upper electrodes 26a and 26b, a diffraction grating 27 with a uniform pitch xcex9, a common lower electrode 28, a structurally built-in TM-mode phase shift region 29 with a length L in the cavity-axial direction, and a stripe waveguide 30. In the active layer 22, a gain for the TM mode is made close to but smaller than that for the TE mode. In a bulk active layer, the gain for the TE mode is larger than that for the TM mode, so a degree of freedom for designing the active layer can be increased.
Where the following notations have respectively the above definitions, the following two relations (especially, relation (4) is indispensable) must be satisfied in the phase shift region 29 in order for the TM mode to receive a xcex/4 phase shift:
(n1TExe2x88x92nTE)xc3x97L=xcexTE/2xe2x80x83xe2x80x83(4),
and
(n1TMxe2x88x92nTM)xc3x97L=xcexTM/4xe2x80x83xe2x80x83(5).
From the two relations, nTE, nTM and L need to be determined such that the following relation (6) is satisfied:
xcex1=(4xc3x97Lxc3x97nTE+xcexTE)/(4xc3x97Lxc3x97nTM+2xc3x97xcexTM)xe2x80x83xe2x80x83(6).
Under the above conditions, a phase shift received by light in the TE mode after a single reciprocative path thereof in the cavity is xcex and the phase of the light after the single reciprocative path coincides with the phase of the original light wave, so the phase shift region 29 does not at all influence the light in the TE mode. Thus, only light waves in the TM mode effectively receive the phase shift, and hence a stable single-mode oscillation can be achieved in the TM mode. Here, since the TE mode never receives the phase shift action, the polarization contention can be made likely to occur by making the gain for the TE mode larger than that for the TM mode. (When only a stable single-mode oscillation in the TM mode is desired, only a gain for the TM mode has to be made sufficient.) Such gain adjustment can be achieved by introducing a relatively small tensile strain into the active layer 22.
Thus, there are structurally provided the phase shift with the polarization dependency for acting on the TM mode only and the gain adjustment for facilitating the polarization contention, so a stable single-mode oscillation in the TM mode and the polarization contention can be obtained. Also in this case, the phase shift region 29 can be elongated, so adverse influences, such as the hole burning from concentrating light in the phase shift portion can be effectively reduced and the operation can be stabilized.
FIGS. 10A and 10B show the third configuration of the laser of the present invention. As illustrated in FIGS. 10A and 10B, the laser includes a substrate 31, an active layer 32, a grating layer 33, a clad layer 34, contact layer (portions 35a and 35b), two upper electrodes 36a and 36b, a diffraction grating 37 with a uniform pitch xcex9, a common lower electrode 38, a structurally built-in first phase shift region 39 with a length Lin the cavity-axial direction, a structurally built-in second phase shift region 40 with a length L2 in the cavity-axial direction, and a stripe waveguide 41. In the active layer 32, gains for TE mode and TM mode are made approximately equal. This can be achieved by introducing an appropriate tensile strain into the active layer 32.
Where the following notations have respectively the above definitions, the following two relations (in this case, relations (7) and (8) are indispensable since a single-mode oscillation is required for each polarization mode) must be satisfied in the first phase shift region 39 in order for the TM mode to receive a xcex/4 shift:
(n1TExe2x88x92nTE)xc3x97L1=xcexTE/2xe2x80x83xe2x80x83(7),
and
(n1TMxe2x88x92nTM)xc3x97L1=xcexTM/4xe2x80x83xe2x80x83(8).
From the two relations, nTE, nTM and L1 need to be determined such that the following relation (9) is satisfied.
xcex11=(4xc3x97L1xc3x97nTE+xcexTE)/(4xc3x97L1xc3x97nTM+2xc3x97xcexTM)xe2x80x83xe2x80x83(9)
Under the above conditions, a phase shift received by light in the TE mode after a single reciprocative path thereof in the cavity is xcex, and the phase of the light after the single reciprocative path coincides with the phase of original light wave in the first phase shift region 39, so the phase shift region 39 does not at all influence the light in the TE mode. Thus, only light in the TM mode effectively receives the phase shift.
Similarly, the following two relations (also in this case, relations (10) and (11) are indispensable because of the above reason) must be satisfied in the second phase shift region 40 in order for the TE mode to receive a xcex/4 phase shift:
xe2x80x83(n2TExe2x88x92nTE)xc3x97L2=xcexTE/4xe2x80x83xe2x80x83(10)
(n2TMxe2x88x92nTM)xc3x97L2=xcexTM/2xe2x80x83xe2x80x83(11)
From the two relations, nTE, nTM and L2 need to be determined such that the following relation (12) is satisfied:
xcex12=(4xc3x97L2xc3x97nTE+2xc3x97xcexTE)/(4xc3x97L2xc3x97nTM+xcexTM)xe2x80x83xe2x80x83(12).
Thus, only the TM mode effectively receives a xcex/4 phase shift in the first phase shift region 39 while only the TE mode effectively receives a xcex/4 phase shift in the second phase shift region 40, so that single-mode oscillations in both the TE mode and the TM mode and the polarization contention can be achieved. Further, the phase shift regions 39 and 40 can be elongated, so adverse influences, such as the hole burning from concentrating light in the phase shift portions can be effectively reduced and the operation can be stabilized.
These advantages and others will be more readily understood in connection with the following detailed description of the preferred embodiments in conjunction with the drawings.